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Atomistic Simulation of Fluids and Environmental Contaminants Confined at Clay Mineral-Water InterfacesAuthor: K Mohan Maruthi Sena Date: 2019-01-10 Report no: IIIT/TH/2019/3 Advisor:Marimuthu Krishnan AbstractThe concentration of atmospheric CO 2 and antibiotic drug effluents from industries continue to increase beyond safety threshold levels in the environment. Mitigation of these contami- nants is necessary to protect the environment from various fatal consequences including global warming and antibiotic resistance caused by them. Expandable clay minerals are emerging as promising candidates for long-term storage of various environmental pollutants and nuclear wastes. The layered aluminosilicate clay minerals permit intercalation of water molecules, simple and complex organic and biomolecules into their interlayer galleries. The intercalated molecular species in the interlayer space cause the clay to swell. The swelling nature of clay is exploited in recent years for storage of pollutants including CO 2 and radioactive materials. The anthropogenic CO 2 is collected from industries and injected into deep geological formations such as saline aquifers and oil reservoirs beneath the earth’s surface at supercritical conditions (323 K, 90 bar). This process is known as geological sequestration of CO 2 . These natural geological formations are sealed on top by cap rocks that ensure leak-proof storage of CO 2 . Cap rocks are made up of swelling clay minerals including montmorillonite and bei- dellite. The CO 2 retention capacity and adsorption/desorption energetics of layered materials are likely to depend critically on the hydration level and the nature of molecular interactions among H 2 O, CO 2 , charge-balancing cations, and the oxide/hydroxide layers. The porosity and permeability of clay minerals are also greatly influenced by intercalated molecules and exchangeable charge balancing cations (CBCs). Thus, molecular-scale understanding of the structure, dynamics, and interfacial energetics of H 2 O/CO 2 binary mixtures confined in the interlayer nanopores is paramount to geological CO 2 storage efforts in clay-rich materials. Antibiotics are commonly used to treat a large variety of infections. These antibiotic drugs are produced in massive scales in India and China. Effluents from antibiotic drug in- dustries in many countries including India are directly released into nearby streams without proper effluent treatment strategies. Recent studies reported that the antibiotic drug con- centration in some streams in India and China is ∼ 100,000-1 million times higher than the prescribed threshold drug levels in the surface water. These drugs ultimately settle in clay 8 layers and enter the human body through food and water causing health hazards including an- tibiotic resistance among people residing around these streams. Due to the dynamic disorder exhibited by intercalates, the atomistic details of physico-chemical properties of these drugs in clay interlayers are not readily amenable to experimental techniques. Molecular dynamics simulations could provide atomistic level insights into the structure, dynamics, and energetics of clay-confined drug molecules. The microbial degradation of plant and animal tissues in the soil gives rise to complex heterogenous soft matter known as the natural organic matter (NOM). NOM plays a crucial role in many geochemical processes in the soil including sorption of toxic metal ions, bio- chemical regulators, radionuclides, organic contaminants, and nutrient transport and they also act as reservoirs of carbon in the soil. Despite significant research efforts in this area of re- search, the molecular structure of NOM is not fully understood because of the fact that the composition of NOM depends on the vegetation, climate, and topography of specific ecosys- tem. Recent experiments describe NOM as heterogeneous supramolecular mixtures of rela- tively small molecules, which are held together by weak non-covalent interactions between various chemical moieties of NOM. NOM-clay interactions play an important role in regu- lating the presence of carbon content in the soil. Much remains to be understood about the atomic-level details of the behavior of NOM at clay-water interfaces. In this thesis, we have employed molecular dynamics simulations and enhanced sam- pling free energy methods to elucidate the atomistic details of the structure and dynamics of H 2 O/CO 2 , antibiotics, and NOM at clay interfaces. In Chapter 3 & 4, we have studied the structure, dynamics, and energetics of H 2 O/CO 2 in smectite clay interlayers. The role of charge balancing cations in the adsorption of CO 2 at mineral-water interfaces is discussed in Chapter 5. In Chapter 6, we have derived force field parameters for antibiotic drug molecules and modeled drug-clay composites using newly generated force field parameters. The inter- actions of different functional groups of NOM, their structure, orientation and affinity towards cations, in clay-water interlayers are discussed in Chapter 7. Full thesis: pdf Centre for Computational Natural Sciences and Bioinformatics |
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